Historically, integrated circuits have been formed on the surface of silicon substrates with conductive pads formed at the periphery of the silicon substrates. Generally, wire bonds are attached to the conductive pads which provide electrical connections from the conductive pads to pads on a package substrate.
Increasing complexity of electronic circuitry included on integrated circuits has required an increase in the number of input and output connections to integrated circuits. The increase in the input and the output connections has required input and output conductive pads to be formed more closely together. That is, the pitch between the conductive pads which are used as inputs and outputs to the integrated circuits has decreased with increased circuit complexity.
The reduction in the pitch between the input and output conductive pads of integrated circuits has required a wire bonding interconnection structure in which the bonding wires are narrower in width and longer in length. However, reduced pitch is not enough and the increasing number of input and output connections requires more rows of connections on the package substrate around the integrated circuit which requires longer bonding wires.
Increasing the length of the bonding wires can cause the bonding wires to suffer electrical and mechanical performance limitations. First, longer bonding wires generally include a greater amount of inductance. Increasing the inductance of a bonding wires reduces the speed of electronic signals which pass through the bonding wires. In addition, increasing the length of bonding wires reduces the mechanical stability of the bonding wires which can vibrate and short, at least temporarily among themselves.
The inductance of the electrical connections can be reduced by replacing the wire bonds with "solder bumps" which are formed on the conductive pads. In addition, solder bumps are generally more mechanically stable than wire bonds. Solder bumps, however, require greater conductive pad spacing or pitch than wire bonds. That is, wire bonds can generally be formed closer together than solder bumps. Therefore, it can be difficult to form solder bumps on conductive pads which were formed with the expectation that wire bonds would be used to provide electrical connections to the conductive pads.
Locating the conductive pads at the interior of the silicon substrate surface rather than the periphery allows the conductive pads to be physically spaced further apart. Therefore, the number of solder bumps electrically interconnected to the integrated circuit substrate can be increased. However, many integrated circuit designs exist in which the conductive pads are located at the periphery of the silicon substrate. The expense to retool the design process of the integrated circuit so that the conductive pads are located at the interior of the silicon substrate surface can be very large. That is, redesigning the integrated circuit so that the conductive pads are located at the interior of the silicon substrate surface can be prohibitively expensive.
It is desirable to have a structure and method for interconnecting conductive pads located at the periphery of an integrated circuit to solder bumps formed at the interior of a silicon substrate surface.